2 leaf wall & soffit design help

Hi, Can anyone point me in the right direction for a guide to dimensions for a 2 leaf wall system please? I need to know the ideal construction and the best distance between the timber frames for each leaf. My plan is to build 2 timber frame walls, 2 layers of either 15mm soundbloc or 2 layers of 12mm plasterboard on the front of the control room side (front of leaf 1), pack with rockwool slabs, 1 layer plasterboard on the back of leaf 1 then pack the remaining space and leaf 2 with rockwool slabs then 3 layers of plasterboard on the live room side (back of leaf 2). I'm wondering if you think this is the best way to do it. I am being dozy I'm sure but I cant seem to find any posts with actual construction details on for this. I'd also very much appreciate it if someone could post me some links to door designs as I need to build a door in to the live room through this wall. Thanks a lot guys...

Dude, What you are describing its a 3 leaf system it MUST be like this: PLASTERBOARD-INSTULATION-FRAME --AIR GAP ---- FRAME-INSULATION-PLASTERBOARD The ammounts of layers of plasterboard its up to you. 2 5/8 reccomended, and the airgap 2" is just fine between the frames Also, please don't be lazy and search the forum. There is a big sticky right on this same forum about a door design

Sorry, I've honestly missed the door post. I've just looked again and found it. Couldnt see the wood for the trees!! Thanks very much for the info. I now understand it a bit better. Is there any recommended way to keep the rockwool in the timber frames so it doesnt fall out in to the air gap? (planning on using 4"x2" timber for the frames btw). Is it best to just cut them a bit too big for the hole and keep them in that way or maybe put some thin strips of wood across the back to keep them in? Would you just use rockwool batts that are like 4" deep (same size as the timber)? I can get them in different thicknesses you see from the supplier so want to make sure I order the right ones. Thanks very much for your reply. Also, Do you have a link to John Sayers soffit design? I've seen it referenced in a few posts but I have searched and cant find it. Thanks again. Ian

Also, I'm going to have the timber frames built mounted on 10mm neoprene against the existing concrete walls to aid isolation. Should the board then be mounted on neoprene against the studs or is this unnecessary as the stud frames will already be isolated? Cheers Ian

Puting neoprene btw the floor and studs isnt a proof solution, since you are already screwing it to the floor to its coupled. It may reduce vibrations tho. There are several neoprene pads that are for board/stud usage but I think its a waste of money for what you get, and since you already have a 2 leaf wall system you dont need as much decoupling from the board to the stud, as opossite if you were dealing with a simple stud wall. However, GreenGlue is a good product for dampening two boards of plasterboard. tho its expensive so leave it at the end if you have enough money left. Trust me that little details eat all of your money! I have spent everal hundred bucks on little details like lights, light bulbs and what not! About the insulation, you'll figure out ! One method is to use metal or nylon wires nailed to the back of the framing, so it holds the instulation together, Also, if you cut the insulation a little bit wider than the stud spacing, it will hold up by itself Soffits! Go to the acoustic sub-forum, you will find plenty of soffit designs with a cup of coffee Cheers

superb, thanks a lot man, I'll check them out. One last question on this post (will prob have to start another one concerning the walls), With the floor being concrete, what is the best way of mounting the studwork to the floor without damaging the dampcourse membrane under the concrete?? Surely screws will go too deep and damage it? Cheers Ian

We use these (bigger version of course): They expand Dyna bolts are much stronger tho, in my case I used the ones above since its not a big building!

Thanks a lot man, you've been a big help :-)

Hey Ian, I got some good news and some bad news. The bad news is: "...against the existing concrete walls to aid isolation." this is about to be the start of a three leaf wall system if you do build two interior frames. Reason being is that the exterior of your existing building is concrete so makes up for one leaf. You only require one more leaf in this air tight system to develop the room-in-a-room design you are going for. The good news is, now you know!

Whoops yeah. I missed that part (was about to go to the studio when writing) But. you got the idea ;)

Aarrgghh, Guys, I dont understand what you mean... If the timber frames are sitting on neoprene and isolated from the structure, how can the side walls make another leaf? I may have confused you with my description... :? Here is a pic of the room before I knocked down the dividing wall and a pic after the wall was taken down. The wall had to be taken down because the door was not central. The back room will be a live room, the front room where I'm standing when taking the pics is the control room. The new dividing wall is to be built where the old one was but come forward a bit to the edge of the chimney breast in the backroom if you see what I mean. Makes sense when looking at the pic. Could you clarify the point regarding the exterior walls acting as a 3rd leaf if you would. Thanks a lot

... how can the side walls make another leaf?

What you said is this:

2 layers of either 15mm soundbloc or 2 layers of 12mm plasterboard on the front of the control room side (front of leaf 1), pack with rockwool slabs, 1 layer plasterboard on the back of leaf 1

OK, so if you put plasterboard on one side if a frame, then plasterboard on the other side of the same frame that is TWO leaves, not one. So the existing wall would be the third leaf. You are counting your "frame with plasterboard on both sides" as one leaf, when in reality it is two. A leaf is not the frame. A leaf is one or more layers of mass with air next to it, regardless of how it is held in place. If you put a couple of layers of plasterboard on a frame, that is one leaf. If you then put a couple layers on the other side of the same frame, that is a second leaf, since there is an air gap between the two sets of plasterboard. Apart from that, 12mm plasterboard is too thin. The resonant frequency is too high, and the mass is too low. Use at least 15 mm, and thicker if you can get it.

If the timber frames are sitting on neoprene and isolated from the structure, how can the side walls make another leaf?

There is no logic to that: a leaf is still a leaf no matter if it is coupled or decoupled from the structure. But apart that, if the frames are sitting on neoprene and isolated from the structure, then you have a dangerous situation! You cannot leave the wall free-standing like that. (At least, where I live you cannot do that. It is against code, since the wall can move if it is not firmly attached to the floor.) Vibration, thermal expansion/contraction, tremors, earthquakes, doors opening and closing.... There are many things that can move a free-floating wall. You need to attach your frames to the floor somehow, such as with nails or bolts though the sole plate into the subfloor. As soon as you do that, your frame is no longer isolated from the structure, since the bolts or nails create a bridge: they couple it. They create a flanking path through the neoprene. Neoprene is good for sealing the wall to the floor, but it cannot fully isolate and decouple the wall, since you need to bolt through it. But anyway, regardless of whether your wall floats on neoprene or not, that has zero effect on whether it counts as one leaf or two. A leaf is a leaf regardless of whether or not it is coupled or decoupled, sealed or not sealed. A leaf is a leaf simply due to being a large massive surface with air behind it. And since the way you describe your frame has plasterboard on both sides of it, that makes two leaves. You then describe another wall, which you refer to as the second leaf, but it is actually the third leaf, since you already have two... "... then pack the remaining space and leaf 2 with rockwool slabs then 3 layers of plasterboard on the live room side (back of leaf 2)...." That is leaf three, not leaf two. - Stuart -

xSpace wrote:

The bad news is: "...against the existing concrete walls to aid isolation." this is about to be the start of a three leaf wall system if you do build two interior frames. Reason being is that the exterior of your existing building is concrete so makes up for one leaf. You only require one more leaf in this air tight system to develop the room-in-a-room design you are going for.

This is what has confused me.There is no existing concrete wall you see as you can see from the pics. This is a dividing wall. I understand your point:

jbassino wrote:

it MUST be like this: PLASTERBOARD-INSTULATION-FRAME --AIR GAP ---- FRAME-INSULATION-PLASTERBOARD

and this makes perfect sense and is the way it needs to be done. The walls will not be totally isolated of course because they will be securely screwed and bolted to the sidewalls, ceiling joists and floor, it is just that the timbers will be sitting on the neoprene foam (which I managed to get really cheap) right around the walls, floor and joist and then screwed and bolted thru that. Obviously it must be attached to the structure otherwise it would be idiotic!!! I know this doesnt give true isolation but it should help and is the best way I can think of doing it. I'm also laying neoprene across the ceiling joists before packing the gaps between the joists (29cm deep - it is a flat roof construction), with 2x100mm rockwool batts and 1x50mm batts which will leave a 4cm approx air gap at the top, then laying the 15mm soundbloc on top of the neoprene. The rockwool will be tight up against the board leaving the airgap at the top (next to the flat roof if you know what I mean).

ian.lb wrote:

Also, I'm going to have the timber frames built mounted on 10mm neoprene against the existing concrete walls to aid isolation. Should the board then be mounted on neoprene against the studs or is this unnecessary as the stud frames will already be isolated? Cheers Ian

ian.lb wrote:

"There is no existing concrete wall you see as you can see from the pics. "

I can only repeat what you have said Ian, they are your words not mine. The biggest issue at present is the distraction that a room in a room can only be completed if it is made of sheetrock as the mass. This is incorrect. If you have an existing wall (AND you do) you must consider this wall and how it will interact within a two leaf system. If you have one solid mass type wall, e.g. solid concrete, brick or stone, then this is one leaf. It is a simple process, don't get confused with all the bold letters and the finality of comments. Get familiar with this linked page: http://www.saecollege.de/reference_mate ... 0Chart.htm What you are trying to achieve you have not completely grasped the basics of, as best as I can tell.

The walls will not be totally isolated of course because they will be securely screwed and bolted to the sidewalls, ceiling joists and floor,

Then I don't see why you are going to all the trouble of building it like that. If you are going to attach your new walls to the existing walls floor and ceiling, then it makes no sense to bother building a two leaf wall at all, since it will be coupled all around the edges. Sound will flank everywhere. If you intend to bolt the walls down like that, then you could probably save yourself some money by just building a single stud frame wall with plasterboard on both sides, since that is about the same level of isolation that you'll get. The entire wall will be coupled anyway, so why bother pretending otherwise? You can't wish away the laws of physics. They will still apply to your room whether or not you want them to.

it is just that the timbers will be sitting on the neoprene foam (which I managed to get really cheap) right around the walls, floor and joist and then screwed and bolted thru that.

Then they will be coupled, and flanking. Once again, neoprene used like that serves only as an airtight seal, it does not serve to isolate the frame from the wall. If you put a bolt through the neoprene, then it is bridged, there is a flanking path (and a really good, solid one, too!) so it is coupled. One more time: neoprene used like that will not isolate or decouple your wall, nor prevent it from flanking: it will only help to seal the air gaps between the frame and the wall. But since you won't even have an MSM system, you don't even need to bother doing that, probably! The wall will be so tightly coupled and flanking that eeven air gaps around the framing won't add much that isn't there already.

I know this doesnt give true isolation but it should help and is the best way I can think of doing it.

That's the problem: You are trying to think of how to do it yourself, intuitively, instead of relying on the science of acoustics, which is not intuitive at first glance. You have knowledgeable people here telling you that what you propose doing is a three leaf wall, that it will flank, that it won't isolate, and telling you how to do it correctly, and then you insist that you are going to carry on doing it your way anyway, because it's the "best that you can think of". So why are you asking for advise, if you just plan to ignore the advise that you get and that you don't like?

Obviously it must be attached to the structure otherwise it would be idiotic!!!

Actually, on the contrary: ATTACHING it to the structure all around would be idiotic (with the exception of the floor). The dividing wall should be attached to the floor ONLY, and to the other inner-leaf walls, and to the inner leaf ceiling. No inner-leaf member should EVER be attached to any outer-leaf member, except at the floor. There must be no mechanical connection at all between inner-leaf and outer-leaf. I absolutely agree with what Brien said:

What you are trying to achieve you have not completely grasped the basics of, as best as I can tell.

That's the way it looks to me too. It seems there are some concepts here, or maybe its the terminology, that you haven't got the hang of yet. All of your inner leaf walls, and the inner leaf ceiling, must be decoupled form the outer leaf, and neoprene pads will not accomplish that. All of your inner leaf walls must have ,ass on only ONE side of the frame, NEVER on both sides of the frame, since that would create a three-leaf wall. In the case of your dividing wall, you described a three-leaf wall very clearly: put drywall on one side of one frame, then put drywall on the other side of that frame, then put drywall on one side of another frame. That makes three leaves. For your other inner-leaf walls (the ones that run parallel to the existing concrete walls) the same holds true. The existing concrete is already your first leaf: You only need one more leaf to complete the MSM wall. That leaf is drywall, and it can go on either side of the frame, but NOT on both sides. Only one side. The same with the ceiling. You already have an outer-leaf ceiling. So you only need to run joists across the tops of your new inner leaf walls WITHOUT attaching them to the outer leaf at an point, then hang your drywall from those joists, either below them or on top of them, but NOT both, and that is your inner-leaf ceiling. Once again, the existing structure IS your first leaf (except for the dividing wall, where you need to build both leaves). You only need ONE additional leaf (one again, except for the dividing wall where you need to build both leaves). There must be ZERO mechanical connections between leaves (except for the floor, which is common to both leaves). And neoprene will NOT decouple anything if you then directly bolt the two structures together, bypassing the neoprene. - Stuart -

Ok, can we start this again? My initial post, I was obviously incorrect, I now know that but didnt at the time. PLEASE IGNORE MY FIRST POST. I'm going to try to keep this simple now. I thank you all for your input on this. Please know that in no way am I ignoring your advice and doing it my own way, if I have given you that impression then I apologise. From what you have told me, if I'm reading this right, whatever gets built should not be attached to the existing walls or ceiling but must be attached securely to the floor. The neoprene underneath the wall along the floor is not a good idea. The neoprene between the frame and the walls / ceiling can be ok to seal it but as soon as you go through it and bolt to the walls, it negates the very thing you are trying to achieve. I had no idea about this and I obviously need to read up further on how to do it as I've never heard of the idea of not attaching in this way. I'll read up on this now I'm aware of it. Please find attached a work in progress sketchup of my room. I know this must be frustrating for you guys because you know what you mean and its probably really simple. I'll post a pic of the existing ceiling construction later and ask your advice on that. It is already constructed and my budget wont stretch to structurally altering the roof / joists....Lets deal with that later. Taking this back to basics, My understanding now is that

jbassino wrote:

PLASTERBOARD-INSTULATION-FRAME --AIR GAP ---- FRAME-INSULATION-PLASTERBOARD

Is the way to build a 2 leaf wall. I'll try to keep my questions simple to avoid confusion.

Soundman2020 wrote:

The existing concrete is already your first leaf: You only need one more leaf to complete the MSM wall.

The wall I am building is a dividing wall. There is no existing dividing wall in place. The external walls of the structure are already in place. Does this mean that before I even start with the dividing wall, I already have one leaf which is the outside wall of the structure?

"PLASTERBOARD-INSTULATION-FRAME --AIR GAP ---- FRAME-INSULATION-PLASTERBOARD Is the way to build a 2 leaf wall. I'll try to keep my questions simple to avoid confusion." It is still an inaccurate statement. MASS - AIR - MASS is a more correct statement. Plasterboard or sheetrock or plywood are not required materials to develop what you want, which is, mass-air-mass. People use what they are familiar with when they make comments, so plasterboard is what the comment above was based on and nothing more, it is not gospel. Mass- air- mass, this is what you are trying to get your head around, so yes, depending on if your existing external wall is solid or not, it is at least your first leaf. If it is a solid wall... "Concrete block walls commonly have wallboard applied to each face as a finishing material; if the air gap behind the wallboard is too small, then the transmission loss can be reduced relative to the unfinished wall. Reduced low frequency TL caused by the mass--air--mass resonance often lowers the STC of otherwise good concrete block walls." http://irc.nrc-cnrc.gc.ca/pubs/bsi/85-3_e.html This is a very good read for a situation such as yours. It may get your mind off* plasterboard:) Good luck, Brien * changed of to off

if I'm reading this right, whatever gets built should not be attached to the existing walls or ceiling but must be attached securely to the floor.

Yep. And it must be attached to the other new walls, and the new ceiling, for stability, rigidity, etc. A single straight wall can't stand on it's own, but it can stand if it has other walls at each end, perpendicular to it, and a ceiling on top.

The neoprene underneath the wall along the floor is not a good idea.

Well, it will help to SEAL the gap underneath, especially if you have a very uneven floor, but it won't help much to FLOAT (decouple) the floor.

The neoprene between the frame and the walls / ceiling can be ok to seal it but as soon as you go through it and bolt to the walls, it negates the very thing you are trying to achieve.

Not really. I wouldn't bother using neoprene there AT ALL. There is no reason to, and you run the risk of jamming it in too tight, compressing it too much, which would cause it to flank. Just leave a decent air gap there. You don't need to attach that dividing wall to the existing walls, since it will be attached to your new side walls. Those joints (new-divider-wall to new-side-walls) must be sealed, of course, but those side walls should be at least a few inches away from the existing (concrete) side walls, so you don't need any neoprene in there: just an air gap.

The wall I am building is a dividing wall. There is no existing dividing wall in place.

Right, but you are ALSO building new SIDE walls, which will be parallel to the existing concrete side walls (or slightly angled, if you splay them). It is those side walls we are talking about: they only need one additional leaf, since the concrete walls provide the other leaf. You need to build both leaves of the divider wall, sure, but the side walls only need one additional leaf, the "inner-leaf", which will be attached to (and support) one leaf of the divider wall.

The external walls of the structure are already in place. Does this mean that before I even start with the dividing wall, I already have one leaf which is the outside wall of the structure?

Not with reference to the divider wall, no. Only with reference to the new side walls. The divider wall needs two leaves in it, but the side walls only need one leaf, since the other leaf is already there. - Stuart -

xSpace wrote:

Not with reference to the divider wall, no. Only with reference to the new side walls. The divider wall needs two leaves in it, but the side walls only need one leaf, since the other leaf is already there.

Aha!! I am enlightened!! Now I know why the conversation got so confusing!! My fault yet again, I have failed to explain to you that due to the narrow dimensions of the room, I am not going to build new side walls per say. The existing walls are solid brick / plaster walls. My plan was to just build the splayed part shown on the sketchup using timber and plasterboard and splay it down to the existing walls, just going as far as where the entry door is. The point of this was to get rid of some of the rectangular dimension of the room. The rest of the walls I was planning to treat by making rockwool filled timber frames to cut down on reflections etc and I was thinking of building bass traps using rockwool in the corners under the window and having a sofa under the window too. The thing is, I dont really need to worry too much about the sound from the control room getting out asI dont have to worry about the neighbours. I'd like to isolate control room from live room as much as poss. apologise I should have explained this earlier.